The present invention relates to an improved process for producing 3-trichloromethyl-5-chloro-1,2,4-thiadiazole. 3-Trichloromethyl-5-chloro-1,2,4-thiadiazole and its derivatives are biocides which are effective against fungi, nematodes and in controlling weeds. It and its derivatives are particularly effective as soil fungicides which function in the soil to protect seeds and growing plants against such pathogenic fungi as Pythium, Fusarium, Rhizoctonia, and Sclerotium. More important, 3-trichloromethyl-5-chloro-1,2,4-thiadiazole is an intermediate in the manufacture of compounds disclosed in Schroeder, U.S. Pat. No. 3,260,725 including but not limited to 3-trichloromethyl-5-ethoxy-1,2,4-thiadiazole.
As disclosed in U.S. Pat. Nos. 3,260,725 and 3,260,588 it is known to prepare 3-trichloromethyl-5-chloro-1,2,4-thiadiazole by adding aqueous caustic to a mixture containing approximately equimolar amounts of trichloroacetamidine hydrochloride and trichloromethanesulfenyl chloride in methylene chloride. Utilizing this procedure yields of 56% are obtained. More recently 3-trichloromethyl-5-chloro-1,2,4-thiadiazole has been prepared by adding excess trichloromethane sulfenyl chloride to a solution of not more than 11% by weight trichloroacetamidine in an organic solvent such as methylene chloride at a temperature of -5.degree.C. to 10.degree.C. followed by addition of caustic at 0.degree.-20.degree.C. to effect ring closure. The solvent is then evaporated and the residue distilled to recover the desired product in yields of up to about 70% theory.
While the more recent prior process was economically feasible and was a substantial improvement over the older process, it had numerous disadvantages which are overcome by the present process. In the prior process trichloroacetamidine concentrations were of necessity limited to about 11% by weight of the organic solvent employed. At higher concentrations a precipitate, trichloroacetamidine hydrochloride, which forms during the addition of trichloromethanesulfenyl chloride renders the reaction mixture so viscous that proper agitation is for all practical purposes impossible. This adversely affects overall productivity of the prior process. We have now found that by controlling the pH of the reaction mixture during addition and reaction of trichloromethanesulfenyl chloride, the formation of trichloroacetamidine hydrochloride is substantially eliminated. We can thus increase the weight ratio of trichloroacetamidine to organic solvent and provide an overall increase in productivity of about 50%.
In the prior process, the reaction mixture was free of added water during the reaction of trichloroacetamidine with trichloromethanesulfenyl chloride to avoid decomposition of the reactants. The only water added to the system resulted from the subsequent use of aqueous caustic to effect ring closure. This was due to the belief that the poor yields obtained in the process described in U.S. Pat. No. 3,260,725 were caused by decomposition of reactants and/or product due to the presence of water during the reaction. To limit such decomposition and improve yields added water was therefore excluded from the system until after completion of the reaction. We have now found that the deleterious effect of added water on yield and productivity is more than offset by controlling the pH of the reaction mixture and limiting the amount of water added to just that necessary to solubilize sodium chloride formed during the reaction.
It has also been found that an unexpected increase in yields may be obtained by carefully controlling the stoichiometry under which the reaction is conducted.
In the prior process disclosed in U.S. Pat. Nos. 3,260,725 and 3,260,588 trichloroacetamidine hydrochloride and trichloromethanesulfenyl chloride were employed in substantially equimolar proportions, i.e., plus or minus about 0.5%. The precise ratio of reactants was not believed to be critical. In practice, however, and in the more recent process referred to above, it was found that yields of about 70% of theory (as opposed to 56% in the patents) could be obtained if a stoichiometric amount or a slight molar excess of trichloromethanesulfenyl chloride was employed. It has now been found that yields in excess of 90% of theory can be obtained by employing a defined molar excess of trichloroacetamidine in combination with pH control. This was highly unexpected in view of our prior experience and at present the rationale behind this unexpected increase in yield is not clear. It is suspected, however, that the excess trichloroacetamidine catalyzes ring closure and thus prevents loss of adduct which occurred in the prior process.
It has also been found that purity of trichloromethanesulfenyl chloride employed in the reaction has a marked effect on trichloroacetamidine conversion. Sulfur chlorides, notably S.sub.2 Cl.sub.2, frequently found in commercially available trichloromethanesulfenyl chloride reduce product yield by about 2% for each 1% present. It is desirable, therefore, although not critical to the invention, to employ trichloromethanesulfenyl chloride of high purity with respect to sulfur chlorides as a starting reactant. But in the absence of such material, which is extremely costly under presently known processes, we have found that the adverse effect of the sulfur chlorides, which exist in presently available trichloromethanesulfenyl chloride, on yields can readily be overcome by factoring the excess of trichloroacetamidine upward in a defined ratio.
Whereas the prior processes were suitable only for batch processing, the present process may be practiced utilizing batch or continuous operation. An additional advantage of the present process is that by utilizing a continuous process it may be conducted at higher temperatures then could be employed in the prior process, eliminating the need for refrigeration.